Lens Bench - Continued
Back focal length. The lens to be tested is mounted in the chuck of the nodal slide, using a light
clamping pressure to avoid straining the lens, and the azimuth of the chuck is adjusted to 0. A mark
with crayon is placed at center of the surface of the lens facing microscope. Then the microscope is
focused on the mark as sharply as possible and the location of microscope on the ways of the bench is
read by means of the vernier and scale. The next step is to move the microscope along the ways until it
is focused on the image of the target. The new location of the microscope on its ways is read on the
scale, again using the vernier. The difference between this reading and the reading previously obtained
is the back focal length of the lens.
Spherical aberration. Spherical aberration can be measured by recording the difference between vernier
scale settings of the observing microscope when focused on the image of target in the central area of
the lens and again when focused on the image of the target in the outer annular area of the lens. When
focusing on the center of the lens, the outer annular area is covered with a ring of opaque paper. When
focusing on the outer annular area of the lens, the central area is covered with a circular disk of opaque
Chromatic aberration. Chromatic aberration may be measured by illuminating the image of the target
with light of a known wave length (using a filter for this purpose), focusing the microscope on the image,
and reading the position of microscope on the scale. The process is repeated with different filters. The
variation of the different scale positions of the microscope will give the effect of chromatic aberration of
the lens with respect to the particular wave lengths used.
Astigmatism. Astigmation can be measured by noting the difference of the settings of the observing
microscope on the vernier scale, when it is focused separately on any two lines of the image of the
target that are perpendicular to each other.
Coma. When the image of a point transmitted through a lens appears comet shaped when viewed
through the observing microscope, the presence of coma is indicated.
Curvature of field. The amount of curvature of the field is determined by measuring the change of focus
required when first focusing on the image through the axial center of the lens, and then focusing on the
image through the outer field of the lens.
Distortion. The presence of distortion is indicated when the image of a line transmitted through a lens
appears curved when viewed through the observing microscope.